Add PooledConnection protocol (#417)

Sources/ConnectionPoolModule/ConnectionPool.swift

@@ -0,0 +1,58 @@
+/// A connection that can be pooled in a ``ConnectionPool``
+public protocol PooledConnection: AnyObject, Sendable {
+    /// The connections identifier type.
+    associatedtype ID: Hashable & Sendable
+
+    /// The connections identifier. The identifier is passed to
+    /// the connection factory method and must stay attached to
+    /// the connection at all times. It must not change during
+    /// the connections lifetime.
+    var id: ID { get }
+
+    /// A method to register closures that are invoked when the
+    /// connection is closed. If the connection closed unexpectedly
+    /// the closure shall be called with the underlying error.
+    /// In most NIO clients this can be easily implemented by
+    /// attaching to the `channel.closeFuture`:
+    /// ```
+    ///   func onClose(
+    ///     _ closure: @escaping @Sendable ((any Error)?) -> ()
+    ///   ) {
+    ///     channel.closeFuture.whenComplete { _ in
+    ///       closure(previousError)
+    ///     }
+    ///   }
+    /// ```
+    func onClose(_ closure: @escaping @Sendable ((any Error)?) -> ())
+
+    /// Close the running connection. Once the close has completed
+    /// closures that were registered in `onClose` must be
+    /// invoked.
+    func close()
+}
+
+/// A connection id generator. Its returned connection IDs will
+/// be used when creating new ``PooledConnection``s
+public protocol ConnectionIDGeneratorProtocol: Sendable {
+    /// The connections identifier type.
+    associatedtype ID: Hashable & Sendable
+
+    /// The next connection ID that shall be used.
+    func next() -> ID
+}
+
+/// A keep alive behavior for connections maintained by the pool
+@available(macOS 13.0, iOS 16.0, tvOS 16.0, watchOS 9.0, *)
+public protocol ConnectionKeepAliveBehavior: Sendable {
+    /// the connection type
+    associatedtype Connection: PooledConnection
+
+    /// The time after which a keep-alive shall
+    /// be triggered.
+    /// If nil is returned, keep-alive is deactivated
+    var keepAliveFrequency: Duration? { get }
+
+    /// This method is invoked when the keep-alive shall be
+    /// run.
+    func runKeepAlive(for connection: Connection) async throws
+}